1. Field of the Invention
The present invention relates to a method of adding to molten steel a low-melting point metal such as lead or bismuth that provides free-cutting properties and/or a low-melting point metal-containing material such as lead oxide or bismuth oxide for the purpose of producing steels containing said low-melting point metals such as, for example, machine structural steels, Al-Si killed steels for use in automotive bodies, and free-cutting steels containing low carbon and sulfur.
For the purpose of description of the present invention in the following pages, the low-melting point metal and/or low-melting point metal-containing material will sometimes be collectively referred to as a source of low-melting point metal.
2. Prior Art
It is known that a source of low-melting point metal such as lead or bismuth that was added to steel exists either independently or as a sulfide to provide significant improvement of the free-cutting property of the steel.
The source of low-melting point metal may be added to steel by the following three methods: (a) the source is added while liquid steel is poured into an ingot making mold; (b) the source is added from above to the liquid steel in a ladle while the steel is agitated with an inert gas such as Ar or N.sub.2 that is blown into the ladle from below, and the steel then is subjected to continuous casting; and (c) the source and the inert gas are injected through a submerged lance into the liquid steel in a ladle, and the steel then is subjected to continuous casting. With the increased use of the continuous casting method, the procedures (b) and (c) have recently come to be employed extensively. However, the method (b) wherein the source of low-melting point metal is dropped from above and ladle is disadvantageous in that the source which usually has a greater specific gravity than iron (Pb, 11.34 or Bi, 9.80&gt;Fe, 7.8) will be dispersed unevenly within the liquid steel and that the low-melting point metal being added will be oxidized to reduce the yield or efficiency of addition of such low-melting point metal source. Because of these disadvantages, the steel into which the source of low-melting point metal is added by the method (b) does not have uniform free-cutting properties and cannot be produced without causing air pollution by lead oxides. The method (c) wherein the source of low-melting point metal is injected into the liquid steel through a submerged lance is free from the aforementioned problems and, hence, is regarded as a favorable means.
One prior art method for injecting a lead-containing substance into molten steel through a submerged lance is described in "Tetsu to Hagane (Iron and Steel)", vol. 68, No. 4, March 1982, '82-S253, The Iron and Steel Institute of Japan. In order to enable gas-carried injection of lead which, in its elemental form, has a specific gravity of as great as 11.34, the apparent specific gravity of lead is reduced and its uniform dispersion in liquid steel is ensured by mixing the lead-containing material with quick lime.
Another version of the method (c) is disclosed in Laid-Open Japanese Patent Publication No. 46311/1985 (laid open to public inspection on Mar. 13, 1985); in this method, a low-melting point metal such as Pb or Bi is mixed with a suflide, either independently or in combination with an oxide thereof, for the same purpose of reducing the apparent specific gravity of Pb or Bi.
The aforementioned prior art methods for adding sources of low-melting point metals involve the following problems.
(i) In the method (c) proposed in "Tetsu to Hagane", ibid., quick lime which usually serves as a desulfurizing flux cannot be added in a large amount because, otherwide, desulfurization occurs while the mixture of lead and quick lime is added to a melt of machine structural steel or Al-Si killed steel for use in automotive bodies, or even to a melt of low carbon, sulfur-containing free cutting steel (which is required to contain as much as 0.310-0.350% S). The low carbon, sulfur-containing steel in which desulfurization has occurred may often fail to comply with the specified sulfur content requirement and will result in poor quality. PA0 (ii) Quick lime is highly hygroscopic, so if it is added in a large amount, the water in the lime will cause an increase in the hydrogen level of the steel. Hydrogen is not desirable in machine structural steels, especially those for use in automotive bodies, and in order to prevent hydrogen pickup, special care must be exercised in handling quick lime or, alternatively, the apparent specific gravity of lead must be reduced by using a substance other than quick lime. The relationship between the addition of quick lime and hydrogen pickup is shown in FIG. 2. PA0 (iii) When oxides of low-melting point metals such as lead oxide and bismuth oxide are added to liquid steel by either of the methods (a) to (c), the oxides are reduced by C, Al, Si or other reducing substances in the melt so as to form inclusions such as Al.sub.2 O.sub.3 and SiO.sub.2. As a result, the Al and Si contents in the steel undergo considerable variation and the overall composition of the liquid steel must be readjusted after the addition of such oxides of low-melting point metals. PA0 (iv) In the methods of (b) and (c), the lead-containing substance is added to the liquid steel within a ladle merely by agitating the liquid steel in a short period of time and the sole control parameter is the rate of addition of lead (see Laid-Open Japanese Patent Publication No. 56562/1984). In the method (c) wherein a powder of lead or lead-containing substance is injected into the steel through a submerged lance, the yield or efficiency of lead addition is high and uniform diffusion of lead in the liquid steel is ensured to provide a steel of good quality. In accordance with the method of the present invention, a more uniform diffusion of lead in the liquid steel is ensured by controlling both the rate at which the lead is added to the liquid steel and the force of turbulence applied to the liquid steel by agitation.
The relationship between the addition of quick lime and the degree of desulfurization is shown in FIG. 1.